1. Field of the Invention
This invention relates to sulfur-modified vegetable oils that have utility as antiwear/antifriction additives for lubricant base oils.
2. Description of the Prior Art
Antiwear/antifriction lubricants typically comprise a base oil that has been blended with any number of additives that enhance the ability of the base oil to withstand the mechanical stresses of interacting working surfaces under boundary lubrication conditions. Most of the lubricants and many of the additives currently in daily use originate from petroleum base stocks that are toxic to environment, making it increasingly difficult for safe and easy disposal. There has been an increasing demand for “green” lubricants [Rhee, I., NLGI Spokesman, 60 (5):28 (1996)] and lubricant additives in recent years due to concerns about loss of mineral oil-based lubricants to the environment and increasingly strict government regulations controlling their use.
Vegetable oils are readily biodegradable, safe to handle, environmentally friendly, non toxic fluids that are also readily renewable resources [Salunkhe, D. K. et al., World Oil Seed Chemistry, Technology and Utilization, Van Nostrand Reinhold, New York, (1992) pp. 1-8; Bockish, M. (ed.) Fats and Oils Handbook, AOCS Press, Champaign, (1998) 838]. The triacylglycerol structure of vegetable oil, which is also amphiphilic in character, give it an excellent potential as a candidate for use as a lubricant or functional fluid [Zaher, F. A. et al., Vegetable oils and lubricants, Grasas Aceites (Seville), 39:235-238 (1988); Willing, A., Chemosphere, 43:89-98 (2001)]. Triacylglycerol molecules orient themselves with the polar end at the solid surface making a close packed monomolecular [Brockway, L. O., J. Colloid Sci., 2:277-289 (1947)] or multimolecular layer [Fuks, G. I., Research in surface forces, A. B. V. Deryagin (ed.) Consultants Bureau, New York (1963) 29-88] resulting in a surface film on the material being lubricated. In addition, the vegetable oil structure provides sites for additional functionalization, offering opportunities for improving on the existing technical properties such as thermo-oxidative, low temperature stability and lubricity. These properties make them very attractive for industrial applications that have potential for environmental contact through accidental leakage, dripping, or generation of large quantities of after-use waste materials requiring costly disposal [Randles, S. J., et al., J. Syn. Lubr., 9:145-161 (1992); Dick, R. M., Process, 41:339-365 (1994)].
Limitations on the use of vegetable oil in its natural form as an industrial base fluid or as an additive relate to poor thermal/oxidation stability [Becker, R., et al., Lubr. Sc., 8:95-117 (1996); Adhvaryu, A., et al., Thermochimica Acta, 364 (1-2):87-97 (2000) and ref. within], poor low temperature behavior [Asadauskas, S., et al., J. Am. Oil Chem. Soc., 76: 313-316 (1999); Adhvaryu, A., et al., Thermochimica Acta, 395:191-200 (2003) and ref. within], and other tribochemical degrading processes [Brophy, J. E. et al., Ann N.Y. Academy Sci., 53:836-861 (1951); Miller, A. et al., Lubr. Eng., 13:553-556 (1957)] that occur under severe conditions of temperature, pressure, shear stress, metal surface and environment. To meet the increasing demands for stability during various tribochemical processes, the oil structure has to withstand extremes of temperature variations, shear degradation and maintain excellent boundary lubricating properties through strong physical and chemical adsorption with the metal. The film-forming properties of triacylglycerol molecules are believed to inhibit metal-to-metal contact and progression of pits and asperities on the metal surface. Strength of the protective fluid film and extent of adsorption on the metal surface dictate the efficiency of a lubricant's performance. It has also been observed that friction coefficient and wear rate are dependent on the adsorption energy of the lubricant [Kingsbury, E. P., ASLE Trans., 3:30-33 (1960)].
The antiwear properties of commercial additives are derived from a variety of elements capable of reacting with the metal surface and establish a stable protective film. Phosphorus, sulfur, nitrogen and zinc constitute the active element in most mineral oil based commercial antiwear additives. However, due to environmental and toxicological considerations, phosphorus may eventually be phased out from usage in the automotive industry because it has been implicated with catalyst deactivation fitted in catalytic converters [Wei, Dan-ping, Lubr. Sci., 7:365-377 (1995)].
Elrod et al. (U.S. Pat. No. 4,181,617) teach an aqueous drilling fluid lubricant consisting essentially of the reaction product of a fatty vegetable oil with 4,4′-thioldiphenol. Exemplary vegetable oils include castor oil, coconut oil, corn oil, palm oil and cottonseed oil.
Baldwin et al. (U.S. Pat. No. 4,559,153) discloses a metal working lubricant comprising a mineral or synthetic oil, and optionally a vegetable oil, and a sulfur-containing carboxylic acid such as n-dodecythioacetic acid and n-butylthioacetic acid. The sulfur-containing additives contemplated by Baldwin et al. are represented by the formula: R—S—R′CO2H.
In an effort to find replacements for sulfurized sperm whale oil, early attempts to sulfurize vegetable oils have resulted in products that displayed a high level of intermolecular cross-linking, and were thus characterized by unacceptable viscosities. Miwa et al. (Proc. Second Int. Conf. on Jojoba and Its Uses, Ensenada, Baja Calif., Norte, Mexico, 1976, pp. 253-264) reports reacting jojoba oil with elemental sulfur. Products from unrefined jojoba oil thickened badly during gear lubricant tests. Similarly, Princen et al. [J. Amer. Oil Chemists Soc., 61:281-89, (1984)] found that sulfurization of the unaltered meadowfoam oil triglyceride oil yielded a factice that was unacceptable as a lubricant additive. Various attempts by Princen et al. to sulfurized wax esters of meadowfoam oil yielded products that had good lubrication properties, but were characterized by one or more deficiencies, such as having a tendency to corrode copper, excessive foaming, unacceptable thermal stability and thicken during gear box tests. Also, Kammann et al. [J. Amer. Oil Chemists Soc., 62:917-23 (1985)] found that sulfurized vegetable triglyceride oils resulted in rubbery products, in some cases even at a 12% sulfur content. Likewise, Wakim (U.S. Pat. No. 3,986,966) teaches that sulfurization of triglycerides yield resinous products mostly insoluble in base oils, and require the addition of nonwax fatty acid methyl esters to improve their solubility.
Erickson et al. (U.S. Pat. Nos. 4,925,581, 4,970,010, 5,023,312, and 5,282,989) are drawn to a lubricating composition consisting essentially of a lubricant base and a lubricant additive. The lubricant additive comprises a mixture of at least two components selected from three classes: the first class of ingredients comprises a triglyceride vegetable oil, a wax ester of the vegetable oil, and a combination thereof; the second class of ingredients comprises: a sulfurized vegetable oil wax ester; a sulfurized triglyceride vegetable oil within the range of from about 25% to about 75% vegetable oil, and from about 25% to about 75% of a wax ester, and a combination thereof; and the third class comprises a phosphite adduct of triglyceride vegetable oil, a phosphite adduct of the vegetable oil wax ester, and a combination thereof. The native vegetable oils contemplated for use by Erikson comprise fatty acids having from about 16 to about 26 carbon atoms and at least one double bond, preferably meadowfoam oil, rapeseed oil or crambe oil.